1. Field of the Invention
The invention relates to a heating device, especially for a motor vehicle, with a burner, to which fuel is supplied from a tank via a fuel line and a pressure reservoir by means of a pump, especially a piston pump, and with an electronically controllable valve means which is inserted into the fuel line and which can be controlled by a control means.
2. Description of the Related Art
A generic prior art heating device is known from the German patent application DE 196 19 861 A1. In this reference, the pressure reservoir is located in the flow path between the pump and the valve means so that the valve means is not able to control charging of the pressure reservoir.
Mainly mechanical or electrically driven piston pumps are used to supply fuel to heating devices. One exception is a high pressure burner. When used to deliver heat, known piston pumps have the disadvantage that the do not work reliably with pressure of roughly 3 bar or more prevailing in the flow pipe. When the fuel is metered via nozzles according to one known proposal, large, uncertain and expensive pressure reducers must be used. Air enters the fuel due to the volumetric delivery and causes problems in the fuel system.
To overcome the latter mentioned defect, for the heating device known from DE 195 26 003 A1, placing a solenoid valve in the fuel line to the burner is proposed. This solenoid valve forms a choke point in the fuel line which offers a considerable resistance to the flowing fuel which increases in excess proportion as the amount of flow increases, while this choke point offers only very little resistance to air or gas or steam. In interaction with a pressure-controlled fuel pump a state is achieved in this way in which problems in the fuel system due to gas bubbles or the like are prevented. The solenoid valve is triggered according to the required burner performance with a defined frequency which corresponds to the stroke frequency of the pump. But this measure does not ensure that the pump will operate reliably at high flow pipe pressures. A similar heating device is known from DE-A1-30 25 283.
DE-A1-21 60 086 discloses inserting into a line between the oil tank and a burner on the pressure side of a pump, a valve which is connected to a chamber which is designed that in the burner line at a certain time, a lower pressure is produced in order to suck bask a certain excess amount of oil from the butner line mouth.
The object of this invention is to devise a heating device which, independently of the pressure made available by the pump, ensures reliable delivery of fuel to the burner.
The invention calls for a combination of the valve means with a pressure reservoir which is used as a buffer and storage reservoir for the fuel which is delivered by a preliminary feed pump (which is part of the motor vehicle or for auxiliary heating by an additional preliminary feed pump of any design) to a downstream burner with a pressure which is fixed by the construction of the pressure reservoir and which is compatible with the downstream burner. This means that the intermittent feed of fuel which has been conventional to date by means of a solenoid valve from the piston pump to the burner is replaced by intermediate storage of fuel in the pressure reservoir and fuel delivery to the burner is exclusively from this intermediate storage. This ensures that the delivery of fuel to the burner is completely independent of the delivery pressure upstream of the valve means so that neither pressure fluctuations from the pump nor an overly high pressure generated by this pump have an adverse effect on the burner function.
To reliably prevent penetration of the pump pressure fluctuations to the burner, it is advantageously provided that only one of the two paths of the valve means, controlled by the control unit, is open at a time specifically, either the first path from the pump into the pressure reservoir in opened or the second path from the pressure reservoir to the burner is opened.
The paths are is controlled in an especially preferred manner such that the first path is closed before the second path is opened. Instead of the latter mentioned measure, the switchover times of the valve means can be kept so short by the corresponding rating of the magnet forces and spring forces thereof, that penetration of the pump to the burner is prevented.
The valve means can basically be designed differently. One preferred embodiment of the valve means is a three-way/two-way solenoid valve or a two-way slide valve for control of the two upstream and downstream flow paths. Alternatively, this two-way valve can also be replaced by a pair of individual valves. Basically, the use of a three-way valve which is controlled accordingly such that penetration from the pump to the burner is prevented is also considered.
To achieve as uniform an amount of delivery over a pressure range which is as wide possible, it is advantageously provided that the pressure reservoir has a soft spring characteristic with a hard limitation can be achieved for example, by using in the pressure reservoir as the storage element with two metal membranes which make contact with one another when the maximum pressure is reached. But also, any other pressure reservoir can be used, such as for example, pistons sealed by sealing lips or roll membranes.
According to one other advantageous development of the invention, the valve means is designed such that in the flowless state, it clears the first flow path from the pump into the pressure reservoir, while it blocks the second flow path from the pressure reservoir to the burner.